Injection and collection diffusion lengths of polycrystalline thin-film solar cells

Polycrystalline thin-film solar cells have three locations of minority carrier recombination: the intra-grain volume, the grain boundaries, and the surface. All three locations contribute to a reduction of the effective diffusion length of the solar cell. Effective diffusion lengths are commonly measured by the internal quantum efficiency technique. Our modeling of the inverse quantum efficiency of polycrystalline thin-film cells shows a first linear region for strongly absorbed light and a second linear region for weakly absorbed light, when plotted against the optical absorption length, An injection diffusion length and a collection diffusion length are defined by the slopes of these linear regimes, respectively. The injection diffusion length is the diffusion length of a hypothetical infinitely thick monocrystalline cell that exhibits the same diode saturation current density, as the polycrystalline thin-film cell. The collection diffusion length equals the diffusion length of an infinitely thick monocrystalline cell that exhibits the short circuit current density of the polycrystalline cell under spatially homogeneous generation.